1. Field of the Invention
The present invention relates to a color image formation apparatus which exposes a photo receptor belt to light from a plurality of exposure light sources to form a color image of a plurality of colors superposed with each other, and more particularly to a photo receptor belt edge detection apparatus for use with a color image formation apparatus of the type mentioned which detects the position of an edge of the photo receptor belt and eliminates a displacement of a light exposure scanning start position on the semiconductor belt to correct the light exposure scanning start position.
2. Description of the Related Art
Conventionally, an image formation apparatus which employs an electrophotographic system is used in printers, copying machines, facsimile apparatus and other like apparatus. A photo receptor member principally with which an image formation process is performed is formed either as a drum type photo receptor member or as a belt type photosensitive member. A color image formation apparatus which prints in multiple colors frequently uses a photosensitive member in the form of a belt, that is, a photo receptor belt in order to facilitate superposition of a plurality of images of different colors on the photosensitive member.
In a color image formation apparatus which employs a photo receptor belt, it is important to adjust the timing of the writing operation so the start positions on the photo receptor belt in the scanning direction is the same for each color image. Otherwise, there will be misalignment of the color images, and the ultimate printed composite color image will be degraded. One way to adjust the timing is to use belt edge position detectors for detecting starting of scanning of laser beams for exposure. The detectors are disposed on the rear face side in the proximity of an edge of the photo receptor belt on the laser scanning starting side such that signals for providing timings at which scanning of image signals are to be started on the photo receptor belt at timings at which the exposure laser beams are intercepted by the photo receptor belt after the exposure laser beams pass the belt edge position detector are outputted from the belt edge position detectors. A color image formation apparatus of the type described is disclosed, for example, in Japanese Patent Laid-Open No. Hei 4-274467.
In the conventional color image formation apparatus, however, the laser beams must be irradiated upon locations outside an image writing range on the photo receptor belt. Therefore, where the color image formation apparatus has such a large size that it handles a paper sheet of the A3 size or the like, it is very difficult to form the laser units for it.
In order to allow a laser beam to be irradiated upon a location outside an image writing range, some color image formation apparatus employs an additional mirror.
However, in order to allow a laser beam to be irradiated directly upon a location outside an image writing range without using an additional mirror, a special lens must be provided. In order to irradiate a laser beam directly upon a location outside an image writing range, a corner portion M of a reflecting surface of a polygon mirror as indicated in FIG. 7 is used. However, reflected light from the corner portion of the polygon mirror is liable to have a low intensity. The special lens mentioned above is a lens designed to restrict the diameter of the laser beam and direct the laser beam so as to be irradiated upon the belt edge position detector at a sufficiently high intensity.
The conventional color image formation apparatus described above is disadvantageous in that it is complicated in structure, difficult to produce and high in production cost because it employs a special lens.
Further, even if it is tried to start writing at a fixed position from an edge of the photo receptor belt after the edge is detected on the basis of timing signals from a clock generator, the writing start positions for the different colors may be displaced from each other by an amount corresponding to one clock pulse unless video clock signals for the different colors are synchronized with each other. Consequently, the conventional color image formation apparatus described above is disadvantageous also in that out-of-alignment of color images may not be eliminated and a color image of a high quality cannot be obtained.